During welding operations, it is often necessary to protect the root of the weld from oxidation as this can lead to weld defects and a reduction in corrosion resistance. This is particularly the case in creep resistant materials, alloy steels, stainless steels and its alloys, nickel and its alloys, and titanium and its alloys. The usual method of protecting the area to be welded is to purge it usually by passing a stream of an inert gas such as argon over the weld area. This limits the availability of oxygen at the weld root to cause oxidation.
However, there are a number of factors which may affect the efficiency of the process and the quality of the weld produced. These factors include the method of damming, the oxygen content of the purge gas and the purge flowrate, all of which can affect the service life of the welded component.
Purging is commonly required when gas tungsten arc welding (GTAW) or plasma processes are used, particularly when stainless steel and alloy steels, nimonics and reactive metals such as titanium or zirconium are being joined.
A common application area is for root runs in circumferential welds in pipe. For pipelines used in the production of electronic components, there is also a requirement to ensure the absence of particles, particularly oxides formed during welding operations.
For steel and nickel alloys, inadequate protection of the rear face of the weld will lead to heavy oxidation and poor penetration bead shape and low corrosion resistance as shown in FIG. 5 (Pitting corrosion potential graph). Further there will be discoloration in the reactive metals and embrittlement.
The problem to be solved is the extended length of time that is needed for oxygen concentration to be reduced from 200,000 ppm to about 10 ppm. The nature of the purging process follows a mathematical power curve of the form Y=AX−b. The nature of this curve is such that the tail of the curve is very long, leading to extended times for reducing the oxygen concentration from 200 ppm to 10 ppm. This time period is controlled by the diffusion mechanism and cannot appreciably be reduced significantly. This time period is dead time for fabricators and manufacturers as no production can continue until the 10 ppm level is reached.
The instant invention reduces this waiting time and utilizes the rapid expansion of liquid cryogenic gases from the liquid phase to the gas phase. The rapid expansion from the liquid to the gaseous state displaces air that is present inside a vessel or pipe to be purged, thereby replacing the air and oxygen present therein with the chosen inert cryogen gas.